YCbCr and standard RGB (sRGB) color spaces are color space standards. In practice, an image device such as a digital camera or a DVD player outputs a YCbCr video signal to a display device, e.g., a digital television (DTV), which then converts the YCbCr video signal to an sRGB format for display. In the CCIR Recommendation 601 specification, Y representing luminosity, and Cb and Cr representing chromaticity are encoded by 8 bits, where Y has a range of 16 to 235, and Cb as well as Cr has a range of 16 to 240. A conversion formula for converting the YCbCr color space to the sRGB color space is described below:
      [                                        R            ′                                                            G            ′                                                            B            ′                                ]    =            [                                    0.00456621                                0.                                0.00625893                                                0.00456621                                              -              0.00153632                                                          -              0.00318811                                                            0.00456621                                0.00791071                                0.                              ]        ·          (                        [                                                                      Y                  601                  ′                                                                                                      C                  B                                                                                                      C                  R                                                              ]                -                  [                                                    16                                                                    128                                                                    128                                              ]                    )      
A red (R) component, a green (G) component and a blue (B) component, respectively ranging from 0 to 1, range from 0 to 255 when represented by 8 bits.
As visual effect requirements of consumers become higher, the YCbCr color space is no longer satisfactory. Therefore, in recent years, an xvYCC (extended video YCC or called x.v.color) color space is developed on the basis of the YCbCr color space. The color range of the xvYCC is about 1.8 times of that of the YCbCr to provide more colorful display. Within the xvYCC color space, Y, Cb and Cr have enlarged ranges of being from 1 to 254 when encoded by 8 bits. When the enlarged ranges are converted according to the conversion formula, R, G and B components have enlarged ranges of being from −17.46 to 277 as illustrated in FIG. 1. However, portions outside the range of 0 to 255, i.e., −17.46 to 0 and 255 to 277, cannot be represented by 8 bits. In the prior art, the portion of −17.46 to 0 is truncated to 0, and the portion of 255 to 277 is truncated to 255. Therefore, extensive color ranges of the xvYCC color space cannot be displayed in the sRGB color space such that the visual effect is deteriorated.